1. Field of the Invention
The present invention relates to a milling cutter such as an end mill.
2. Description of the Related Art
Heretofore, as a milling cutter having a plurality of peripheral cutting edges, such as an end mill, various forms are known. However of these, those where the shape of the transverse cross-section (the cross-section perpendicular to the axial direction) is a regular hexagon or a regular square are easy to manufacture due to the simple shape. Hence development of these has been actively promoted (refer for example to Japanese Unexamined Patent Application, First Publication Nos. Hei 3-26413 or 4-53615).
As one example of these, an end mill with a transverse cross-section of a regular hexagon is shown in FIG. 5 and FIG. 6, while an end mill with a transverse cross-section of a regular square is shown in FIG. 7 and FIG. 8. The end mills in these figures have a shank 10, and a cutting part at a tip end thereof. On this cutting part is formed a plurality of peripheral cutting edges 12 along a cylindrical peripheral face, and each peripheral cutting edge 12 is twisted in a helix.
Looking at the cutting part in the transverse cross-section shown in FIG. 6A and FIG. 8A, the respective vertices of the polygons constituting the transverse cross-section constitute the peripheral cutting edge 12, the vicinity of the front side in the tool rotation direction (the cutting direction) of the peripheral cutting edge 12 constitutes a rake face 14 of the peripheral cutting edge 12, and the vicinity of the rearside in the tool rotation direction constitutes a relief face 16. That is to say, the rake face 14 of one peripheral cutting edge 12 is also used a relief face of the adjacent peripheral cutting edge 12 on the front side in the tool rotation direction of that peripheral cutting edge 12.
Consequently, with the end mill having the transverse cross-section of a regular hexagon shown in FIG. 6A, as shown in FIG. 6C, with a rake angle xcex1 of each of the peripheral cutting edges 12 of xe2x88x9260xc2x0, a relief angle xcex5 of 30xc2x0, the wedge angle xcex2 (the apex angle constituting the peripheral cutting edge 12 in transverse cross-section) becomes 120xc2x0, while with the end mill having the transverse cross-section of a regular square shown in FIG. 8B, as shown in FIG. 8C, with a rake angle xcex1 of each of the peripheral cutting edges 12 of xe2x88x9245xc2x0, a relief angle xcex5 of 45xc2x0, the wedge angle xcex2 (the apex angle constituting the peripheral cutting edge 12 in transverse cross-section) becomes 90xc2x0.
In the tip of the cutting part, as shown in FIG. 6B and FIG. 8B, when viewed from the tool tip side, there is formed bottom edge rake faces 19 extending in the tool diameter direction so that the apex angles of the two peripheral cutting edges 12 are respectively evenly divided. These bottom edge rake faces 19 extend in approximately parallel directions to the tool axis, with the tips thereof constituting bottom edges 18, and tool tip faces located on the rear side in the tool rotation direction of the bottom edges 18 become bottom edge relief faces 20. Consequently, a wedge angle xcex3 of each bottom edge 18 (the apex angle subtending the bottom edge 18 viewed from the tool tip side) become approximately half of the wedge angle xcex2 of the peripheral cutting edge 12, and with the end mill shown in FIG. 6B, xcex3=60xc2x0, while with the end mill shown in FIG. 8B, xcex3=45xc2x0.
Comparing the aforementioned regular hexagon section end mill and the regular square section end mill, for the regular hexagon section end mill, the size of the wedge angle xcex2 of the peripheral cutting edge is larger (xcex2=120xc2x0) than for the regular square section end mill, and to that extent there is a strengthwise advantage. However on the other hand, the rake angle xcex1 of the peripheral cutting edge is much smaller at xe2x88x9260xc2x0. Therefore there is the disadvantage of inferior sharpness. Conversely, with the regular square section end mill, the rake angle xcex1 of the peripheral cutting edge is larger at xe2x88x9245xc2x0. Therefore, compared to the regular hexagon section end mill, sharpness is excellent. However since the relief angle xcex5 is also large, the wedge angle xcex2 of the peripheral cutting edge is small (xcex2=90xc2x0). Hence it is difficult to maintain sufficient cutting edge strength, and to that chipping is likely to occur.
That is to say, with the conventional end mill where the transverse cross-section is a regular polygon (the same for other end mills), there is the characteristic that, if the number of vertices, that is the number of peripheral cutting edges, is increased the strength of the peripheral cutting edges is increased, however the sharpness deteriorates, while on the other hand, if the number of peripheral cutting edges is reduced, the sharpness improves, however the strength of the peripheral cutting edges is reduced. Hence there is the problem that it is extremely difficult to improve the sharpness and at the same time to maintain the cutting edge strength.
In particular, as shown in FIG. 6B and FIG. 8B, in an end mill where the bottom edge rake faces 19 are formed so as to equally divide the apex angles of the peripheral cutting edges 12, and the bottom edges 18 are formed in the tips thereof, the wedge angle xcex3 of each bottom edge 18 further becomes approximately half the wedge angle xcex2 of the peripheral cutting edge 12. Hence insufficiency of cutting edge strength at this bottom edge 18 is serious.
The present invention takes in to consideration such a situation, with the object of providing a milling cutter which can provide for both an improvement in sharpness and a retention of cutting edge strength, while making the transverse cross-section an easily manufactured polygon shape.
As a means for solving the above problems, the present invention, in a milling cutter having a plurality of peripheral cutting edges around a circumferential direction, and with a transverse cross-section shape of a polygon, is constructed such that the polygon is a convex polygon with one more apex point existing between apex point pairs constituting the peripheral cutting edges with an apex angle greater than that of the apex points, and a longer side of the sides on either side of each of the peripheral cutting edges constitutes a rake face, while a shorter side constitutes a relief face.
According to this construction, there is the easily manufactured transverse cross-section shape polygon. However instead of the conventional regular polygon, this is an irregular convex polygon with an apex point existing between apex point pairs constituting the peripheral cutting edge, with an apex angle greater than that of the apex points. Furthermore, the long side of these sides constitutes the rake face, while the short side constitutes the relief face. Therefore, due to the difference in length of these sides (in other words the difference in the apex angles), as the rake angle of each peripheral cutting edge is increased, the relief angle is reduced, and to that extent the wedge angle of the peripheral cutting edges can be kept large. That is to say, it is possible to simultaneously realize both an improvement in sharpness and a retention of cutting edge strength.
For example, in the case where the milling cutter according to the present invention has four peripheral cutting edges (that is, the transverse cross-section shape is an octagon), then with the rake angles of each of the peripheral cutting edges greater than xe2x88x9260xc2x0 and less than xe2x88x9245xc2x0 (xe2x88x9260xc2x0 with the conventional regular hexagon milling cutter), the apex angle of each of the peripheral cutting edges in transverse cross-section can be an angle greater than 120xc2x0 and less than 135xc2x0 (120xc2x0 with the conventional regular hexagon milling cutter). That is to say, while keeping the edge angle of the peripheral cutting edge the same as or greater than for the conventional regular hexagon milling cutter, a greater rake angle can be set.
According to the present invention where the edge angle of the peripheral cutting edge can be kept large in this way, then in a milling cutter (typically an end mill) where a bottom edge is formed not only on the peripheral cutting edge but also on the tip end, a more noticeable affect is realized.
For example, in a milling cutter (end mill) where a bottom edge rake face is formed along a tool diameter direction so that an apex angle of the peripheral cutting edge viewed from the tool tip end is divided, and a bottom edge is formed at the tip end of this bottom edge rake face, the wedge angle of the bottom edge becomes an angle which divides the edge angle of the peripheral cutting edge, and since this becomes even smaller than the edge angle of the peripheral cutting edge, then particularly at the bottom edge chipping is likely to occur. Therefore conventionally in order to raise the strength of the bottom edge, there was the situation where the bottom edge must be formed at a position away from the tool tip central position. However by keeping the edge angle of the peripheral cutting edge large as with the present invention, the wedge angle of the bottom edge can also be kept large. Hence even if the bottom edge is formed at a position close to the tool tip central position, the strength of this cutting edge can be kept high. Consequently, also at the tool tip center part it is possible to maintain a good sharpness.
Furthermore, if the construction is such that the bottom edge rake face is directed rearward from the bottom cutting edge and extends approximately parallel to the tool axial direction, and extends in the rearward direction in a gradually separating direction from the peripheral cutting edge, the gap of the bottom edge rake face and the peripheral cutting edge fulfills a so called honing function (chamfering) so that a chipping prevention effect is further enhanced.